linux-kernel - Re: [PATCH] fs: namespace: Simplify function commit

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Message-ID: <20150113052801.GH22149@ZenIV.linux.org.uk>
Date:	Tue, 13 Jan 2015 05:28:01 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: namespace: Simplify function commit_tree()

On Tue, Jan 13, 2015 at 01:08:06PM +0800, Jiang Liu wrote:
> Simplify function commit_tree() in fs/namespace.c, it also improves code
> readability.

Huh?

>  static void commit_tree(struct mount *mnt, struct mount *shadows)
>  {
>  	struct mount *parent = mnt->mnt_parent;
> -	struct mount *m;
> -	LIST_HEAD(head);
>  	struct mnt_namespace *n = parent->mnt_ns;
>  
>  	BUG_ON(parent == mnt);
> -
> -	list_add_tail(&head, &mnt->mnt_list);
> -	list_for_each_entry(m, &head, mnt_list)
> -		m->mnt_ns = n;
> -
> -	list_splice(&head, n->list.prev);
> -
> +	mnt->mnt_ns = n;
> +	list_add_tail(&mnt->mnt_list, &n->list);

And who will set ->mnt_ns on other mounts reachable via ->mnt_list from mnt?
Not to mention putting them on n->list...

What you seem to have missed is that list_add_tail(&head, &mnt->mnt_list)
is not the same thing as list_add_tail(&mnt->mnt_list, &head).  We are
not ripping mnt->mnt_list from whatever list it was on and insert it into
a previously empty one in head.  We take an unanchored cyclic list (all
elements being some_mnt->mnt_list) and anchor it at head, then go through
all elements of the original list and move them all at once into the
end of n->list.
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